Attempting to override a method in plugin

MyPhase.scala

class MyPhase extends PluginPhase {
  import tpd._

  val phaseName = "overrider"

  override val runsAfter = Set(Pickler.name)

  override def transformTypeDef(tree: TypeDef)(implicit ctx: Context): Tree =
    if tree.isClassDef && !tree.rhs.symbol.isStatic then  // only look at classes & traits, not objects
      
      // 0. Get a FreshContext so we can set the tree to this tree. (for '{} later)
      implicit val fresh = ctx.fresh
      fresh.setTree(tree)
      QuotesCache.init(fresh)
      implicit val quotes:Quotes = QuotesImpl.apply()  // picks up fresh
      import quotes.reflect.*

      val s3ReflectionClassSymbol = getClassIfDefined("co.blocke.scala_reflection.Skip_Reflection").asInstanceOf[ClassSymbol]
      if tree.symbol.getAnnotation(s3ReflectionClassSymbol).isDefined then

        // 1. Set up method symbol, define parameters and return type
        val toJsonSymbol = Symbol.newMethod(
          Symbol.spliceOwner,
          "toJson",
          MethodType(
            List("sb","config"))( // parameter list
              _ => List( // types of the parameters
                TypeRepr.of[StringBuilder],
                TypeRepr.of[SJConfig],
                ), 
              _ => TypeRepr.typeConstructorOf(classOf[Unit])  // return type
            ),
            Flags.Override,  // NOTE: Setting override flag
            Symbol.noSymbol
          )

        // 2. Get our class' Symbol (for ownerhsip reassignment)
        val classSymbol = tree.tpe.asInstanceOf[quotes.reflect.TypeRef].classSymbol.get

        // 3. Define our method definition (DefDef) using our method symbol defined above
        val toJsonMethodDef = DefDef(
          toJsonSymbol, 
          {
            case List(List(sb: Term, config: Term)) => 
              given Quotes = toJsonSymbol.asQuotes
              Some({
                quoted.Expr.ofList(List(
                  '{ println("Hello") },
                  '{ println("World") }
                ))
              }.asTerm.changeOwner(toJsonSymbol))
          }
        ).changeOwner(classSymbol)

        // 4. Add toJsonMethodDef to tree and return
        val classDef = tree.asInstanceOf[ClassDef]
        val cd = ClassDef.copy(classDef)(
          name = classDef.name,
          constr = classDef.constructor,
          parents = classDef.parents,
          selfOpt = classDef.self,
          body = classDef.body :+ toJsonMethodDef
        )
        cd.asInstanceOf[dotty.tools.dotc.ast.tpd.Tree]
      else 
        tree
    else
      tree
}

/*
Produces this error when plugin used to compile a class:

Exception in thread "sbt-bg-threads-1" java.lang.ClassFormatError: Duplicate method name "toJson" with signature "(Lscala.collection.mutable.StringBuilder;Lco.blocke.scala_reflection.SJConfig;)V" in class file com/foo/Person
	at java.base/java.lang.ClassLoader.defineClass1(Native Method)
	at java.base/java.lang.ClassLoader.defineClass(ClassLoader.java:1013)
	at java.base/java.security.SecureClassLoader.defineClass(SecureClassLoader.java:150)
	at java.base/java.net.URLClassLoader.defineClass(URLClassLoader.java:524)
	at java.base/java.net.URLClassLoader$1.run(URLClassLoader.java:427)
	at java.base/java.net.URLClassLoader$1.run(URLClassLoader.java:421)
	at java.base/java.security.AccessController.doPrivileged(AccessController.java:712)
	at java.base/java.net.URLClassLoader.findClass(URLClassLoader.java:420)
	at sbt.internal.ManagedClassLoader.findClass(ManagedClassLoader.java:102)
	at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:588)
	at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:521)
	at com.foo.Person$.apply(Main.scala:13)
	at com.foo.Main$package$.hello(Main.scala:35)
	at com.foo.hello.main(Main.scala:27)
	at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:104)
...
*/